This invention relates to a method of fabricating a thin film on a substrate and an apparatus for the same, and more particularly to a method of decomposing a raw gas and fabricating a thin film containing an element mainly consisting of the raw gas on the surface of the substrate and an apparatus for the same.
A film-forming method known so far which comprises the steps of decomposing a raw gas by glow discharge at reduced pressure and forming a film containing an element mainly consisting of the decomposed raw gas on the surface of a substrate involves a plasma chemical vapor deposition (abbreviated as "CVD") apparatus of the induced coupling or capacitive coupling type. The conventional plasma CVD apparatus comprises the steps of taking a raw gas into a decompressed reaction chamber; supplying high frequency power to a high frequency coil surrounding the reaction chamber or a pair of mutually facing electrodes provided in the reaction chamber; producing plasma by glow discharge out of the raw gas held in the reaction chamber; and depositing a film containing an element mainly consisting of the raw gas on the surface of a substrate placed in the reaction chamber.
Recently it was proposed to deposit a film over a broad surface of a substrate by decomposing a raw gas containing, for example, silicon by means of discharge. An amorphous silicon film which is deposited on a substrate can be used as a photosensitive material. Silane (SiH.sub.4) is a most widely accepted as a silicon-containing raw gas. Accompanied with plasma flashing induced by the grow discharge of silane, various kinds of radicals and ions are produced. The those active species are deposited on a substrate surface. Thus, an amorphous silicon film containing hydrogen is fabricated on the surface of the substrate.
When, however, an amorphous silicon film is deposited particularly over a broad area, the above-mentioned conventional plasma CVD apparatus has the drawback that the deposition takes a long time. To eliminate the difficulties resulting from the long hours of silicon film deposition, it has been contemplated to apply a larger amount of high frequency power in the glow discharge in order to accelerate the deposition of the silicon film. However, a mere increase in high frequency power fails not only to deposit a silicon film with a uniform thickness over a broad area, but also to produce an amorphous silicon film of high quality. The reason why a high quality amorphous silicon film cannot be produced is that beams of light emitted from the plasma during the formation of the silicon film are projected on the silicon film in the process of being grown on the substrate surface, thereby breaking the bonding of the surface portion of an unstable active silicon film. Among the beams of light emitted from the plasma, ultraviolet rays in particular have extremely high energy, tending to produce a large amount of free radicals in the silicon film.
Thus, an increase in high frequency power only results in the greater intensity of emitted light beams and consequently the failure to form a high quality silicon film. The failure to produce a high quality silicon film is generally supposed to result from the fact that emission of a strong light results in a decline in the photoconductivity or dark-conductivity (referred to as "the Steabler-Wronski effect"). This phenomenon results from an increase in difficulties caused by emission of intense light beams. Quantitatively, this phenomenon is understood to arise from a reduction in the photoluminescence due to a dangling bond or an increase in the spin density in electron spin resonance (abbreviated as ESR).
With the conventional plasma-CVD apparatus, therefore, power consumption must be reduced in order to form a high quality silicon film, thereby making it impossible to accelerate the deposition of a silicon film.